The preparation of acetic anhydride by contacting in the liquid phase a mixture comprising methyl acetate and/or dimethyl ether and methyl iodide with carbon monoxide in the presence of a carbonylation catalyst at elevated pressures and temperatures has been reported extensively in the patent literature. See, for example, U.S. Pat. Nos. 3,927,078; 4,046,807; 4,115,444; 4,252,741; 4,334,884; 4,374,070; 4,430,273; 4,559,183; 5,003,104; 5,292,948 and 5,922,911 and European Patents 8396; 87,869; and 87,870. These patents disclose that the reaction rate can be increased if the catalyst system includes a promoter such as certain amines and quaternary ammonium compounds, phosphines and phosphonium compounds and/or inorganic compounds such as alkali metal salts, e.g., lithium iodide. Normally, both the reaction (process) mixture and the crude product are substantially anhydrous, homogeneous liquids comprising a solution of the reactants and catalyst components in an inert solvent such as acetic acid. Thus, the crude, liquid product obtained from such acetic anhydride processes typically comprises a mixture of acetic anhydride and acetic acid as a result of the use of acetic acid as a process solvent.
Acetic acid may be coproduced in the process by feeding methanol and/or water to the production system, e.g., by feeding methanol and/or water to a process recycle stream containing acetic anhydride and/or to the carbonylation reactor. See, for example, U.S. Pat. No. 5,380,929, U.S. Pat. No. 6,130,355, EP-00087869-B1 and EP-00087870-B1. U.S. Pat. No. 4,374,070 discloses adding methanol to an acetic anhydride-containing recycle stream.
Generally speaking, an acetic anhydride production line or a co-production line includes a reaction section, a purification section, light ends recovery and a catalyst recovery system. In the reaction section, methyl acetate and/or dimethyl ether, methyl iodide and carbon monoxide are contacted with a Group VIII metal catalyst such as a rhodium catalyst in a homogenous stirred liquid phase reaction medium in a reactor. The reaction section also generally includes a flash vessel coupled to the reactor which flashes a draw stream in order to remove crude product from the reaction section. The crude product is fed to a purification section which includes generally a light ends or stripper column, iodine removal, and auxiliary separation/purification, as discussed hereinafter. In the process, various non-condensible vent streams containing light ends, notably methyl iodide, carbon monoxide and methyl acetate are generated and fed to the light ends recovery section. These vent streams are scrubbed with a solvent to remove the light ends which are returned to the system or discarded.
There is an ongoing need for better energy and raw material efficiency as is seen, for example, in United States Patent Application Publication No. US 2007/0287862 of Kline et al. entitled “Production of Acetic Acid and Mixtures of Acetic Acid and Acetic Anhydride”. This reference discloses a process for the production of acetic acid or mixtures of acetic acid and acetic anhydride in a carbonylation process wherein a mixture comprising methyl acetate and/or dimethyl ether and methyl iodide is contacted in the liquid phase with carbon monoxide in the presence of a carbonylation catalyst at elevated pressures and temperatures. Methanol, water, or a mixture thereof is added to an acetic anhydride-containing stream within a flash evaporation zone to convert some or all of the acetic anhydride to acetic acid and optionally methyl acetate and to provide heat for the evaporation of a portion of the product effluent produced by the carbonylation process.
In accordance with the present invention, there is provided an improved carbonylation system with staged reaction and pre-flash removal of light ends to increase productivity and purification efficiencies.